Device and method for equalizing the charges of individual, series-connected cells of an energy storage device

ABSTRACT

A device and a method for compensating charges of serially connected individual cells of an energy storage device includes a DC/DC converter which taps power from the energy storage device or an additional energy source, charges a capacitor of an intermediate circuit by way of the tapped power, inverts the voltage thereof in a DC/AC converter, converts the alternating voltage into an intermittent direct current via an AC bus and a double capacitor by way of a rectifier, and charges the cell with the intermittent direct current at the lowest cell voltage.

The invention relates to a device and method for equalizing the chargesof individual cells of an energy storage device disposed in series, inparticular of capacitors of a double-layer capacitor connected inseries, as used for example in vehicle electrical systems.

Double-layer capacitors have proved to be the most practical technicalsolution for storing and supplying short bursts of high power in avehicle electrical system, for example during acceleration assistance(boost) for the internal combustion engine by an integratedstarter-generator operating as an electric motor or during theconversion of motive energy to electrical energy by the integratedstarter-generator operating as a generator during the regenerativebraking process (recuperation).

The maximum voltage of an individual cell of a double-layer capacitor islimited to around 2.5V to 3.0 V, such that for a voltage of 60V forexample—a typical voltage value for a double-layer capacitor used in a42V vehicle electrical system—around 20 to 26 individual capacitors haveto be connected in series to form a capacitor stack.

The varying self-discharge rates of the individual cells mean that acharge inequality develops over time in the capacitor stack, ultimatelymaking the double-layer capacitor unusable, if the charges are notequalized.

If the discharge curve is extrapolated to periods of weeks to months, asare relevant for the motor vehicle, the existing problem becomes clear.FIG. 1 shows an example of the scatter range of the capacitor voltagesfor a double-layer capacitor (capacitor stack) with 18 cells(capacitors) over time. The scatter range (between maximum and minimum)illustrated in FIG. 1 shows the degree to which the self-discharge ofthe individual cells within a capacitor stack can fluctuate over time.

A simple charge equalizing operation, for example by slightlyovercharging the capacitor stack, as with a lead-acid battery, ishowever not possible with a double-layer capacitor.

One option known internally within the company is to monitor the voltageof each individual cell by means of a separate electronics system(operational amplifier and voltage divider R1/R2) and, when apredetermined maximum value U_(ref) is reached or exceeded, to bringabout a partial discharge by means of a connectable parallel resistanceR_(byp) (FIG. 2). The cell then discharges by way of the parallelresistance R_(byp) and its voltage U_(C) drops back below the maximumvalue. If the voltage drops below the maximum value by a predeterminedvoltage value, the parallel resistance R_(byp) is disconnected again.

In the passive state such a circuit uses little energy but chargeequalizing is achieved by charge release (energy loss in the parallelresistance R_(byp)). This variant should be used expediently where acapacitor stack is mainly operated close to the maximum voltage, forexample in the case of the power supply to emergency generating units.

The concept is however restricted in that the charging current into thecapacitor stack must be smaller than the discharging current of thecharge equalizing circuit, as otherwise it would not be possible toprevent the overcharging of individual capacitors when charging themodule. Also the equalizing system cannot be switched on externally, butis only activated when the predetermined voltage threshold is exceeded.During operation in a motor vehicle however this precise state is notachieved over quite a long period. Such a charge equalizing operationresults in the long term in a lack of symmetry in the capacitor stack.It has already been possible to demonstrate this by taking measurementsin a test vehicle.

To summarize, such a circuit arrangement has the followingdisadvantages:

-   -   no feedback to a higher-order management system, when a cell has        exceeded the maximum voltage (for example U_(C)>2.5V),    -   no feedback to determine whether the cell voltages are equal and        the capacitor stack is therefore equalized,    -   the equalizing operation is only activated when the maximum        voltage is exceeded,    -   energy is converted to heat by resistors during the equalizing        process,    -   in the case of large currents of up to approx. 1 kA, as occur        during the vehicle function recuperation (regenerative braking),        as described above, such a charge equalizing structure is not        possible.

It is known from EP 0 432 639 B2 that where there are a number ofbatteries connected in series a charge equalizing operation can bebrought about between a weakly charged battery and the set of otherbatteries, in that a comparison circuit and a charge circuit (having arectangle function generator) as well as a diode, transformer andinterrupter are provided for each individual battery of the batterystack.

With such a device, operating as a flyback converter according to theisolating transformer principle (FIG. 3), energy is tapped from thestack as a whole and then fed back into the most discharged battery.

This outlay may be justified for two or three batteries but it isdefinitely too high for a stack of twenty or more batteries/capacitorcells.

Alternatively another energy source—perhaps an additional battery—canalso be used here, with the result that the circuit can also be used tocharge the capacitor stack slowly—see also DE 102 56 704 B3.

This form of charge equalizing can also be implemented at any time,regardless of whether the individual capacitor has reached a maximumvoltage, such that a dangerous charge inequality cannot develop in thecapacitor stack.

In this process charges are simply displaced. In the long term no energyis tapped from the stack or converted to heat. This makes the conceptparticularly attractive for motor vehicle applications, as sufficientenergy must still be present in the vehicle electrical system, evenafter quite a long vehicle stoppage, to ensure that the vehicle startssuccessfully in a reliable manner.

One disadvantage of this embodiment is however that the secondary sideof the flyback transformer requires a large number of terminals. In thecase of a capacitor stack with for example 25 individual cells, asrequired for the 42V vehicle electrical system, this means 50 terminals.For the purposes of technical implementation this would require aspecial winding body, which is not commercially available. Also anychange in the number of cells in the stack requires adjustment of thetransformer. This is to be expected however, as the further technicaldevelopment of double-layer capacitors has led to an increase in thepermitted maximum voltage from generation to generation andcorrespondingly fewer individual capacitors are required for a givenmodule voltage.

The wiring arrangement from the transformer to the capacitor cells isalso complex, as every contact in the stack has to be connectedseparately. In the example above this means 26 lines, in so far as therectifier diodes are disposed on the transformer; otherwise it wouldmean 50 lines. These lines are also subject to high-frequency voltagepulses from the switching processes of the flyback converter and requirespecial EMV suppression measures.

A further aspect is the method for operating the flyback converter.Commercially available control circuits (switching controller ICs)operate almost exclusively with a fixed switching frequency. Thecharging of the magnetic store (storage inductivity or transformer)takes place in the one phase, while the discharge or energy transfer tothe output circuit takes place in the other phase of the clock pulse.This is particularly expedient when a direct current element also has tobe transferred as well as the switched current (continuous operation).Generally every effort is made to avoid a switching gap—in other wordsthe time period during which the magnetic storage element remains fullydischarged—as oscillations then tend to occur more frequently and thestorage characteristics of the magnet core are not used optimally. Theoscillations have their origin in the resonant circuit, which comprisesstorage inductivity and winding capacitance, and the fact that theresonant circuit is stimulated at the start of the switching gap and isnot damped by any ohmic load.

In the present application continuous operation is however not possible,as when the magnetic store is continuously recharged, before completelydischarging in each instance, saturation of the core material cannot beavoided.

The object of the invention is to create a device with a simplifiedstructure which can be used to achieve automatically controlledoperation for equalizing the charges between the individual cellsconnected in series with little technical outlay.

The object of the invention is also to create a method for operatingthis device.

According to the invention this object is achieved by a device accordingto the features of claim 1 and a method for operating said deviceaccording to the features of claim 11.

Where at least two energy storage devices (cells) are connected inseries, the energy required to equalize the stored charges is fed by wayof an alternating current bus (AC bus) to the cell with the lowest cellvoltage in each instance.

Advantageous developments of the invention will emerge from thesubclaims.

The interfacing and isolation of the cells is effected by way ofcapacitors according to the invention.

Installation is simple due to the bus system. The individual cells aresupplied by way of one or two AC bus lines. Only a few, low-costcomponents are required for the circuit and these are essentiallystandard components.

The equalizing process can be activated at any time. Such activation canfor example be effected by a control device, which determines theactivation time based on operating parameters of a motor vehicle, inparticular of an internal combustion engine and/or a starter-generator.

The capacitor stack can be recharged by way of the equalizing circuit.It is thus possible to charge up a series circuit of empty cells againfrom a further energy source, for example rendering a motor vehicle thathas been stopped for a long time once again capable of starting.

The system as a whole can be easily expanded and is therefore readilyscalable.

The circuit arrangement is particularly suitable for integration intothe stack of cells of an energy storage device connected in seriesand/or into the housing of the individual cells or the energy storagedevice as a whole.

Particularly suitable energy storage devices in this instance aredouble-layer capacitors, also referred to as super-caps or ultra-caps.

Exemplary embodiments according to the invention are described in moredetail below with reference to a schematic drawing, in which:

FIG. 1 shows the scatter of the capacitor voltages of different cells ofa double-layer capacitor over time,

FIG. 2 shows a known circuit arrangement to achieve charge equalizing inenergy storage devices,

FIG. 3 shows a further known circuit arrangement to achieve chargeequalizing in energy storage devices,

FIG. 4 shows a block circuit diagram of an inventive charge equalizingcircuit,

FIG. 5 shows an exemplary embodiment of a charge equalizing circuit and

FIG. 6 shows a further exemplary embodiment of a charge equalizingcircuit.

FIGS. 1 to 3 have already been described above.

A block circuit diagram of an outline circuit for equalizing the chargesof cells of an energy storage device according to the invention is shownin FIG. 4. A first converter 1 generates a direct voltage. This directvoltage is inverted by way of a second converter 2 with a pulsefrequency of 50 kHz for example and this alternating voltage is appliedto an AC bus 4. Bus here refers to a system of conductors (cables,copper rails, etc.).

The cells Z₁ to Z_(n) of the double-layer capacitor DLC connected incircuit are connected to this bus 4 by way of a coupling capacitor and arectifier 3 respectively. The coupling capacitors C_(K) are used forisolation purposes and their charge is partially reversed by thealternating voltage.

FIG. 5 shows a first exemplary embodiment of an inventive circuitarrangement for equalizing the charges of cells Z₁ to Z_(n) of adouble-layer capacitor DLC. The voltage U_(DLC) dropping across theseries circuit of the individual cells Z₁ to Z_(n) of the double-layercapacitor DLC is fed to a DC/AC voltage converter 1—for example acurrent-regulated step-down converter—by way of a first switch S1. Anenergy source, for example a battery B, can be connected additionally oralternatively to a DC/DC voltage converter 1 by way of a second switchS2.

The DC/DC voltage converter 1 is in turn connected electrically to aninput of a DC/AC converter 2, which in this exemplary embodiment has anintermediate circuit capacitor C_(Z) and two transistors T1 and T2connected as a half-bridge. The intermediate circuit capacitor C_(Z) caneither be charged by the double-layer capacitor DLC by way of the switchS1 or by the battery B by way of the switch S2. The output of this DC/ACvoltage converter 2 between the two transistors T1 and T2 is connectedto an AC bus 4, which in turn has a coupling capacitor CK₁ to C_(Kn) forthe cell Z₁ to Z_(n) assigned to it.

A rectifier 3, in this instance comprising two diodes D_(xa), D_(xb)respectively, is disposed between each coupling capacitor C_(Kx) (x=1 .. . n) and the cell Z_(x) assigned to it. The diodes—D_(xa)—connect theterminal of the coupling capacitor C_(Kx) with the terminal having thehigher potential (hereafter referred to as the “positive terminal”) ofthe assigned cell Z_(x) in each instance, said terminal of the couplingcapacitor C_(Kx) facing away from the AC bus, and the diodes D_(xb)connect said terminal to the terminal having the lower potential(hereafter referred to as the “negative terminal”) of said assigned cellZ_(x).

The diode D_(xa) is hereby poled from the coupling capacitor C_(Kx)toward the positive terminal of the cell Z_(x) in the through-flowdirection, while the diode D_(xb) is poled from the negative terminal ofthe cell Z_(x) toward the coupling capacitor C_(Kx).

The DC/AC voltage converter 2, in this exemplary embodiment comprising ahalf-bridge T1, T2, supplies a rectangular alternating voltage at itsoutput between the two transistors T1 and T2, it being possible for thecoupling capacitors C_(K1) to C_(Kn) to transfer said rectangularalternating voltage to the individual cells Z₁ to Z_(n).

Different capacitor types can be used for the coupling capacitors.However the capacity, frequency and internal loss resistance of thecapacitor must be aligned. Incorrect alignment would result in too greata charge reversal in the coupling capacitors, thereby having an adverseaffect on the selectivity of the equalizing circuit in the long term.

The current is rectified again by way of the connected rectifier 3(diodes D_(1a), D_(1b) to D_(na), D_(nb)) and fed to the cells Z₁ toZ_(n) as a charging current.

To be able to achieve the equalizing of charges at the capacitor cellsZ₁ to Z_(n) of a double-layer capacitor DLC connected in series, energymust be tapped from those cells Z₁ to Z_(n) having the highest voltageand be fed back to the cells at which the lowest voltage is present,such that these cells are charged.

The circuit can be sub-divided into three sub-circuits. The first partis a current source, which is advantageously in the form of a DC/DCswitching controller 1. During a charge equalizing operation energycomes from the double-layer capacitor DLC itself or—during a chargingprocess—from a second energy source, for example a battery B. Thisenergy is fed to an intermediate circuit capacitor C_(z) of the secondsub-circuit. All known variants of the DC/DC switching controller 1 arepossible; it is advantageously configured as a step-down switchingcontroller comprising transistors, inductor and freewheeling diode (notshown).

In addition to the intermediate circuit capacitor C_(z) the secondsub-circuit 2 has a bridge circuit, in this instance a half-bridge,comprising the two transistors T1 and T2, which is supplied from theintermediate circuit capacitor C_(K), and the output of which isconducted by way of the AC bus 4 to all coupling capacitors C_(K1) toC_(Kn). It generates an alternating voltage, in relation to referencepotential GND (ground).

The third sub-circuit, the rectifier 3, is present once for each cell Z₁to Z_(n). It converts the alternating current to a pulsing directcurrent flowing through the cells.

The charge equalizing process is described by way of example for a cellZ_(x) (where x=1 to n), which is to have the lowest cell voltage U_(Zx)in this exemplary embodiment.

The coupling capacitor C_(Kx) is charged in the negative phase of thealternating voltage signal (transistor T2 conducting current) by thelower diode D_(xb) to the lower potential (at the negative terminal ofthe cell) of the cell Z_(x)—minus the conducting-state voltage of thediode D_(xb).

If the alternating voltage signal then increases the potential to asufficient degree (transistor T1 conducting current), current flows fromthe intermediate circuit capacitor C_(Z) by way of transistor T1, the ACbus 4, the coupling capacitor C_(Kz) and the diode D_(xa) through thecell Z_(x) and through all the cells, whose positive terminal has asmaller potential to reference potential GND than the positive terminalof the cell Z_(x) to be charged, in this instance the cells Z_(x+1) toZ_(n), and from there back to the intermediate circuit capacitor C_(Z).

In the next negative phase of the alternating voltage signal (transistorT1 conducting current again) the current flows in the reverse directionthrough the cells, whose positive terminals have a smaller potential toreference potential GND than the positive terminal of the cell Z_(x) tobe charged, in other words the cells Z_(n) to Z_(x+1), and now throughdiode D_(xb) and the intermediate circuit capacitor C_(Kx). The currentcircuit is closed by way of the AC bus 4 and the current-conductingtransistor T2.

A pulsing direct charging current therefore results in the cell Z_(x),while all the cells Z_(x+1) to Z_(n), whose positive terminal has asmaller potential to reference potential GND, experience an alternatingcurrent.

The pulsing direct current can only flow into the cell Z_(x) with thesmallest cell voltage U_(Zx) and charges this cell first, until saidcell reaches the next highest cell voltage of a further cell. Thepulsing direct current is then distributed to these two cells, until itreaches the cell with what is then the next highest cell voltage, etc.The charges of the entire capacitor stack, in other words all the cellsof the double-layer capacitor DLC, are thus equalized.

The energy, with which the respective cell Z_(x) of the double-layercapacitor DLC is charged, comes from the intermediate circuit capacitorC_(z), which sets itself automatically to an appropriate voltage U_(cz),due to this load on the one hand and the constant recharging on theother hand. This automatically means that the cell, at which thesmallest voltage drops, receives the most energy, while cells (in thisinstance Z₁ to Z_(x−1) and Z_(x+1) to Z_(n)), at which a higher cellvoltage currently drops, receive no energy.

Top-quality high-capacitance coupling capacitors and diodes with lowconducting-state voltages are particularly suitable here.

The inventive circuit has the following function groups:

-   -   a current-regulated step-down converter 1, supplying the        h-bridge 2,    -   a self-clocking h-bridge 2,    -   an AC bus 4, to which the individual cells are connected to tap        the energy,    -   coupling capacitors C_(K1) to C_(Kn) for isolation and energy        transfer purposes, and    -   a rectifier 3 with diodes D_(1a), D_(1b) to D_(na), D_(nb) to        rectify the alternating current, which charges the cell having        the smallest voltage in each instance.

FIG. 6 shows a further exemplary embodiment of the inventive circuitarrangement with a full bridge and a (Graetz) rectifier in a two-phasevariant. Here too the cell Z_(x) is the one with the lowest cell-voltageU_(Zx).

Parts with identical functions have the same reference characters hereas in FIG. 5.

The circuit of the exemplary embodiment with two phases operates in asimilar manner to the circuit of the exemplary embodiment describedabove and shown in FIG. 5 with a half-bridge and one phase. There arehowever certain advantages here which have to be offset against theadditional outlay.

The exemplary embodiment according to FIG. 6 has as its DC/AC voltageconverter 2 a full-bridge circuit with two half-bridges, comprising afirst and second transistor T1-T2 or third and fourth transistor T3-T4,each being connected to a bus line 4.1, 4.2. Each bus line is suppliedwith energy by way of the half-bridge assigned to it.

The bus line 4.1 is connected to the cells Z₁ to Z_(n) connected inseries, in each instance by way of a coupling capacitor C_(K1a) toC_(Kna) and a rectifier circuit comprising two diodes D_(1a), D_(1b) toD_(na), D_(nb) respectively.

The bus line 4.2 is connected to the cells Z₁ to Z_(n) connected inseries, in each instance by way of a coupling capacitor C_(K1b) toC_(Knb) and a rectifier circuit 3 comprising two diodes D_(1c), D_(1d)to D_(nc), D_(nd) respectively.

For the cell Z_(x) for example this means: the bus line 4.1 connected tothe half-bridge T1-T2 is connected by way of the coupling capacitorC_(Kxa) on the one hand by way of the diode D_(xa) conducting currenttoward the cell to the positive terminal of the cell Z_(x) and on theother hand by way of the diode D_(xb) conducting current toward thecoupling capacitor to the negative terminal of the cell Z_(x).

The bus line 4.2 connected to the half-bridge T3-T4 is also connected byway of the coupling capacitor C_(Kxb) on the one hand by way of thediode D_(xc) conducting current toward the cell to the positive terminalof the cell Z_(x) and on the other hand by way of the diode D_(xd)conducting current toward the coupling capacitor to the negativeterminal of the cell Z_(x).

The two rectifiers D_(xa), D_(xb) and D_(xc), D_(xd) therefore operateparallel to the cell Z_(x). The circuit for all the other cells Z₁ toZ_(x−1) and Z_(x+1) to Z_(n) looks similar.

A significant advantage with two phases is that there is no alternatingcurrent through the cells that are not actually involved, which arecurrently not charged, in other words all the cells, whose positiveterminal has a smaller potential to reference potential GND but highercell voltages U_(Z) than the cell Z_(x) (in other words through thecells Z_(x+1) to Z_(n) here).

In this exemplary embodiment the two half-bridges operate in phaseopposition, in other words when the transistors T1 and T4 conductcurrent in the first phase, the transistors T2 and T3 arenon-conducting; this is reversed in the second phase: here thetransistors T2 and T3 conduct current, while the transistors T1 and T4are non-conducting.

In the first phase a current flows from the intermediate circuitcapacitor C_(Z) by way of transistor T1 into the bus 4.1, by way ofcoupling capacitor C_(Kxa) and diode D_(xa) through the cell Z_(x) andback by way of diode D_(xd), coupling capacitor C_(Kxb), the bus 4.2 andtransistor T4 to the intermediate circuit capacitor CZ.

In the second phase a current flows from the intermediate circuitcapacitor C_(z) by way of transistor T3 into the bus 4.2, by way ofcoupling capacitor C_(Kxb) and diode D_(xc) through the cell Z_(x) andback by way of diode D_(xb), coupling capacitor C_(Kxa), the bus 4.1 andtransistor T2 to the intermediate circuit capacitor C_(z).

The recharging current of the one coupling capacitor C_(Kxa) and thedischarging current of the other coupling capacitor C_(Kxb) compensatefor each other.

The step-down converter 1 taps the energy from the entire capacitorstack, comprising the individual cells Z connected in series, in otherwords the double-layer capacitor DLC. Energy can optionally be fed tothe system by way of an additional switch S2.

The voltage at the respective AC bus increases until it corresponds tothe lowest cell voltage plus one (exemplary embodiment according to FIG.5) or two diode voltages (exemplary embodiment according to FIG. 6).This achieves very efficient recharging of the most discharged cell.

The circuit as a whole does not require any complex or expensiveindividual components.

The structure of the AC bus 4 or 4.1 or 4.2 means that the system caneasily be expanded. Additional energy storage devices Z_(n+1) can easilybe connected to the bus and superfluous ones can easily be removed.

The charge equalizing circuit can also be used to equalize the chargesof other energy storage devices, for example batteries connected inseries.

These circuit arrangements (DLC, rectifier diodes, coupling capacitorsand bus line(s)) can be integrated both in the housing, which enclosesthe individual cells, or in a common housing. This provides a compactunit, which only has three or four terminals.

1-13. (canceled)
 14. A device for equalizing charges of series-connectedindividual cells of an energy storage device, comprising: a DC/DCconverter connected by way of a first switch to a terminal of the energystorage device; a DC/AC converter connected to an output of said DC/DCconverter, said DC/AC converter containing an intermediate circuitcapacitor and a bridge circuit; at least one AC bus connected to anoutput of said DC/AC converter; and a series circuit of at least onecoupling capacitor and a rectifier connected between said at least oneAC bus and each cell of the energy storage device.
 15. The deviceaccording to claim 14, wherein said coupling capacitor has a firstterminal connected to said AC bus and a second terminal, and saidrectifier has a first diode conducting current from said second terminalof said coupling capacitor to a positive terminal of the respectivelyassigned cell and a second diode conducting current from a negativeterminal of the cell to said second terminal of said coupling capacitor.16. The device according to claim 14, which comprises a second switchenabling a connection of said DC/DC converter to a further energysource.
 17. The device according to claim 14, wherein said DC/DCconverter is a current-regulated step-down converter.
 18. The deviceaccording to claim 14, wherein said bridge circuit of said DC/ACconverter is a single-phase half-bridge with two transistors connectedin series, and disposed parallel to said intermediate circuit capacitor.19. The device according to claim 14, wherein said bridge circuit ofsaid DC/AC converter is a multi-phase bridge with each phase being ahalf-bridge comprising two transistors connected in series, and disposedparallel to said intermediate circuit capacitor.
 20. The deviceaccording to claim 14, wherein the energy storage device is adouble-layer capacitor.
 21. The device according to claim 14, whereinthe energy storage device comprises a series circuit of storagebatteries.
 22. The device according to claim 18, wherein said bridgecircuit of said DC/AC converter is a self-clocking circuit.
 23. Thedevice according to claim 19, wherein said bridge circuit of said DC/ACconverter is a self-clocking circuit.
 24. The device according to claim14, wherein the energy storage device, said rectifier, said couplingcapacitors and said AC bus are commonly integrated in a common housing.25. A charge equalization method, comprising: providing the deviceaccording to claim 14; feeding, with the DC/DC converter supplied by theenergy storage device or a further energy source, a current to theintermediate circuit capacitor, causing a voltage for charging the cellsat the intermediate circuit capacitor; inverting the voltage with theDC/AC converter and feeding a rectified pulsing charging current by wayof the at least one AC bus, the respectively assigned couplingcapacitors and the diodes of the rectifier to a cell of the energystorage device with a lowest cell voltage.
 26. The method according toclaim 25, wherein the DC/AC converter is a single-phase DC/AC converter,and a charging current for the cell with the lowest cell voltage flowsin a positive phase from the intermediate circuit capacitor by way ofthe current-conducting first transistor, the AC bus, the couplingcapacitor, and the diode to the cell and from the cell by way of allcells, having a positive terminal with a smaller potential to referencepotential than a positive terminal of the cell to be charged, and by wayof reference potential back to the intermediate circuit capacitor, andthe charging current flows in a negative phase in a reverse directionfrom the now current-conducting second transistor through the cells ofthe energy storage device whose positive terminal have a smallerpotential to reference potential than the positive terminal of the cellto be charged, through the diode, the coupling capacitor and the AC busback to the second transistor.
 27. The method according to claim 25,wherein the DC/AC converter is a multi-phase DC/AC converter, and acharging current for the cell with the lowest cell voltage flows in thefirst phase from the intermediate circuit capacitor by way of the firsttransistor, the first AC bus, coupling capacitor and diode, through thecell and back by way of the diode, the coupling capacitor, a second ACbus and a fourth transistor to the intermediate circuit capacitor, andin a second phase flows from the intermediate circuit capacitor by wayof a third transistor, the AC bus, the coupling capacitor and the diode,through the cell and back by way of the diode, the coupling capacitor,the AC bus and the second transistor to the intermediate circuitcapacitor.